Fix interval division and multiplication, before:

	test=> select '4 months'::interval / 5;
	   ?column?
	---------------
	 1 mon -6 days
	(1 row)

after:

	test=> select '4 months'::interval / 5;
	 ?column?
	----------
	 24 days
	(1 row)

The problem was the use of rint() to round, and then find the remainder,
causing the negative values.